The problem of monitoring ink level in all types of high speed printers such as ink-jet printers with ink reservoirs has been variously addressed. So-called back pressure indicators require a plurality of complex seals within the pen cartridge assembly and are therefore relatively expensive and tend to be unreliable. Other ink volume indicators rely on measurement of ink bulk conductivity. The conductivity of the ink is difficult to control and there is the likelihood that future ink improvements could make such a system obsolete.
There have also been attempts to count the "dots" or drops from a given pen. The counters, actuators and sensors needed for such systems make them relatively expensive. Furthermore, accuracy is compromised by the need to assume an average drop volume for all pens. Interruptions such as caused by removal of a pen/cartridge assembly or shut-down of the printer are a further source of unreliability since the record of the number of drops fired from the ink jet since the last update is likely to be lost.
Prior art known to applicants comprises U.S. Pat. Nos. 4,196,625; 4,202,267; 4,371,790; 4,415,886; 4,551,734; 4,587,535; 4,626,874; 4,719,475; and 4,935,751; and pending application Ser. No. 07/423,158 filed Oct. 18, 1989 in the names of John Mohr, et al for a CAPILLARY RESERVOIR INK LEVEL SENSOR and now owned by the assignee of the present invention.
With the exception of U.S. Pat. No. 4,935,751 which is discussed below, and U.S. Pat. No. 4,587,535 which discloses a system of the pressure sensing type, all of the above patents describe monitoring systems which rely on measurement or detection of ink conductivity.
U.S. Pat. No. 4,935,751, owned by the assignee of the present invention, discloses a mechanical level sensor for an ink bag which employs a rigid plate secured to one side of a collapsible ink bag wherein one end of the strip is visible through a window in the ink bag housing. Although the position of the edge of the indicator strip is indicative of the remaining amount of ink in the bag, an "empty" indication appears although an amount of useable ink remains in the bag.
Also of interest are prior co-pending U.S. patent applications Ser. No. 07/929,615 filed Aug. 12, 1992 by Kaplinsky, et. al entitled COLLAPSIBLE INK RESERVOIR STRUCTURE AND PRINTER INK CARTRIDGE and Ser. No. 07/928,811 filed Aug. 11, 1992 by Khodapanah, et. al entitled INK PRESSURE REGULATOR FOR A THERMAL INK-JET PRINTER, both owned by the assignee of the present application.
Further developments of this collapsible bag technology are disclosed in the U.S. patent applications filed on the same day as this application titled METAL COVER ATTACHMENT TECHNIQUE FOR THERMAL INKJET PEN by inventors Dale D. Timm, Jr., et. al Ser. No. 07/994,810; RIGID LOOP CASE STRUCTURE FOR THERMAL INK-JET PEN by inventors David W. Swanson, et. al Ser. No. 07/994,808; and TWO MATERIAL FRAME HAVING DISSIMILAR PROPERTIES FOR THERMAL INK-JET CARTRIDGE by inventors David W. Swanson, et. al Ser. No. 07/994,807 all owned by the assignee of the present invention.
None of the foregoing references provides a simple and inexpensive ink volume indicator. In fact, even if the enclosure is transparent, visual observation of ink in a collapsible ink bag reservoir is not reliable since the collapse of the reservoir as ink is used does not produce direct level change although volume change is, of course, occurring.
One example of an improved ink volume indicator is disclosed in U.S. patent application Ser. No. 07/717,735, filed Jun. 19, 1991, U.S. Pat. No. 5,539,353, entitled SPRING-BAG PRINTER INK CARTRIDGE WITH VOLUME INDICATOR filed by David S. Hunt and W. Bruce Reid and assigned to the assignee of the present invention. The cartridge disclosed in that application basically comprises a rectangular housing containing a flexible bag of ink, an ink filter and a print head which receives ink from the filter. A spring inside of the bag of ink urges its flexible walls apart from each other thus maintaining a negative or sub-atmospheric pressure in the reservoir which is overcome as ink is emitted from the print head. The manner in which the invention advances the state of the art in respect to ink volume monitoring in a collapsible reservoir assembly will be evident from the following description of the invention.